Logistic with Baseline
An S-shaped step between two plateaus, symmetric in x. Use it for melting and denaturation curves, activation curves and other transitions that start and end at nonzero levels.
Fit the example dataStart with your own data
Example: a simulated DNA melting curve, 31 points with x and y uncertainties, opened as a new copy you can edit.
Parameters
- Height of the step, from the low-x plateau D to the high-x plateau A + D. It is negative for a falling curve.
- Steepness, in inverse units of x. Automatic starting values make it positive.
- Midpoint: the x at which y is halfway between the plateaus.
- Plateau at low x, when B is positive.
Tips
- The step rises from 10% to 90% of its height over an x interval of about 4.4/B.
- With D fixed at 0, this is Logistic. For a step that is symmetric in log x, such as a dose–response curve, use Logistic 4PL (Hill).
- Include points on both plateaus. If one is not reached, enter its value as a Guess and check Fix.
- In electrophysiology this is the Boltzmann function, with the half-activation point C and slope factor 1/B.